1. Field of the Invention
The present invention relates to a thermal head and a manufacturing method for the thermal head.
2. Description of the Related Art
There has been conventionally known a thermal head which is used in a thermal printer to be installed frequently in a small-sized information equipment terminal typified by a small-sized handy terminal, and which performs printing on a heat-sensitive recording medium by selectively driving some of a plurality of heating resistors based on printing data (for example, see Japanese Patent Application Laid-open No. 2007-83532).
For improving efficiency of the thermal head, there is a method of forming a cavity portion in a substrate that supports the heating resistors. This cavity portion functions as a hollow heat-insulating layer, whereby, among an amount of heat generated in the heating resistors, an amount of heat transferred downward, which is transferred toward the substrate, is reduced. Meanwhile, an amount of heat transferred upward, which is transferred to the above of the heating resistors, is increased. Thus, efficiency of energy required at the time of printing can be improved.
In a thermal head described in Japanese Patent Application Laid-open No. 2007-83532, an upper substrate and a supporting substrate, which are made of the same material such as glass, are bonded onto each other, whereby an integral substrate is constituted. A concave portion is provided in any one of the upper substrate and the supporting substrate, and the upper substrate and the supporting substrate are bonded and integrated with each other so as to close the concave portion, whereby a cavity portion is formed in an inside of the integral substrate. In the integral substrate as described above, the upper substrate functions as a support member that supports the heating resistors and the like, and also functions as a heat storage layer that stores heat from the heating resistors. Accordingly, a thickness dimension of the upper substrate is important in terms of performing quality control of the thermal head. In particular, when plate thinning treatment, surface treatment, or the like are performed to the upper substrate, variations may occur in the thickness of the upper substrate. Therefore, it is necessary to perform the quality control for the thermal head so as to eliminate the variations in the thickness of the upper substrate.
However, the upper substrate is integrated with the supporting substrate, and in addition, the heating resistors, a protective film, and the like are formed on a surface of the upper substrate. Therefore, the completed thermal head has a problem in that the thickness of only the upper substrate can be no longer measured. In the case of measuring the thickness of the upper substrate of the completed thermal head, the thickness must be measured after decomposing the thermal head.